Background: To develop an empirically informed support measure for workers, we examined mental health distress and its risk factors among prefectural public servants who were affected by the Great East Japan Earthquake and faced a demanding workload in the midterm of the disaster. Methods: We conducted a self-administered health survey of all public servants in the Miyagi prefectural government two and seven months after the Great East Japan Earthquake (3743 workers, 70.6% of all employees). We calculated odds ratios (ORs) and 95% confidence intervals (CIs) for mental distress (defined as K6 score ≥10) in the domain of disaster-work-related stressors, work-related stressors, and disaster-related stressors. Results: Among those with better levels of workplace communication, the only factor that increased the risk of mental distress was not taking a non-work day each week (adjusted OR 2.55, 95% CI 1.27–5.14). Among those with poorer levels of workplace communication, in addition to not taking a non-work day each week (adjusted OR 3.93, 95% CI 3.00–5.15), handling residents’ complaints (adjusted OR 1.55, 95% CI 1.00–2.42), having dead or missing family members (adjusted OR 2.87, 95% CI 1.53–5.38), and living in a shelter more than two months after the disaster (adjusted OR 2.80, 95% CI 1.32–5.95) increased the risk of mental distress. Conclusions: All workers should be encouraged to take a non-work day each week. Among workers with poor workplace communication, special attention should be given to those who handle residents’ complaints, have lost a family member(s), and are living in a shelter for a prolonged period of time.